Rechargeable electrochemical power supply

ABSTRACT

This invention relates to an improvement in zinc-air and other rechargeable electric storage cells whereby the zinc-electrode surface is mechanically activated allowing the cell to be recharged very rapidly without the growth of dendrites on the zinc plates.

ljfiliied States Patent 91 Eisner Feb. 13, 1973 1 RECHARGEABLEELECTROCHEMICAL POWER SUPPLY [75 Inventor:

[73] Assignee: Norton Company, Troy, N.Y.

[22] Filed: July 15, 1970 [21] Appl. No.: 54,887

Steve Eisner, Schenectady, NY.

[52] U.S.,Cl. ..136/86 A, 136/140, 136/141 [51] Int. Cl. ..H01m 29/04,l-lOlm 31/02 [58] Field of Search ..l36/86, 140, 141; 204/36, 227,

[56] References Cited UNITED STATES PATENTS 3,440,098 4/1969 Stachurski..136/86 A FOREIGN PATENTS OR APPLICATIONS 17,469 8/1929 Australia..204/227 Primary Examiner-Allen B. Curtis Attorney-Hugh E. Smith andHerbert L. Gatewood 5 7 ABSTRACT This invention relates to animprovement in zinc-air and other rechargeable electric storage cellswhereby the zinc-electrode surface is mechanically activated allowingthe cell to be recharged very rapidly without the growth of dendrites onthe zinc plates.

10 Claims, 1 1 Drawing Figures PATENTED 3.716.413

SHEETlUF 7 INVE'NTOR Steve Eisner BY/ I ATTORNEY PATENTEU FEB l 3 I973SHEET 2 OF 7 INVENTOR t fi ATTORNEY PATENTED FEB I 3|975 SHEET 30F 7///////////fi7////?/ ////fi/ w////// INVENTOR fiteve isrzer PATENTES E I3 B75 SHEET 5 OF 7 INVE'NTOR 51:96 v M/ ATTORNEY RECHARGEABLEELECTROCHEMICAL POWER SUPPLY FlELD OF THE INVENTION Zinc-air storagecells have been proposed for use in the storage of electricity forindustrial vehicles, electric motors, electric automobiles and unusedoutput from atomic energy plants. Zinc-air cells are presently availablehaving commercially feasible discharge properties. The present inventionrelates to improvements in zinc-air cells and other rechargeable cellswhereby in addition to the advantageous discharge properties, the cellscan also be rapidly recharged without any detrimental effect to thecells ability to store and discharge electrical energy.

DESCRlPTlON OF PRIOR ART Zinc-air cells have been proposed for suchapplications as storage cells in industrial vehicles, electric motorcars and storage of periodic unused output from atomic energy plants.Employment of zinc-air cells for these applications has seriously beenhampered because the zinc-air cell, which has advantageous discharge andstorage properties, lacks the ability to be quickly recharged. When azinc-air cell is subjected to the normal recharging cycle, such as usedin recharging lead-acid storage batteries, zinc is electrodeposited onthe zinc electrode in the form of columnar type growths calleddendrites. The process is sometimes referred to as treeing becausethe'electrodeposit formed on the zinc electrode resembles branches oftrees. During the recharging cycle, these'dendrites grow out from thezinc electrode on which they have formed and come into contact with theother positively-charged electrode which results in a short-circuitedcell.

The rate of formation of dendrites increases as the battery charges.During the initial charging period a generally level deposit of zinc isobtained but as the charging period continues, the dendrites begin toform. This phenomena has made it extremely difficult to plate arelatively thick zinc deposit from alkaline electrolytes in prior artbatteries.

in addition to the dendrite growth problem, zinc cells have been subjectto failure by electrode shape change Shape change is the loss ofgeometric surface area due to sluffing off and redistribution of zincduring charge and discharge cycling. Shape change results in a gradualloss of capacity and voltage with cycling and eventuallythe cell becomesno longer useful.

Zinc-air cells can be mechanically recharged by removing the used zincelectrode from the cell, replacing the used zinc electrode with a newone and replacing the cell electrolyte with fresh electrolyte solution.This process is inconvenient and time consuming. The caustic solutionused as the electrolyte is not amenable to being conveniently stored andchanged when needed and it can cause serious burns if it comes incontact with a persons skin or clothing.

Several attempts to provide separators between the electrodes to inhibitdendrite growth in these cells have been reported. French Patent1,177,402, issued Apr. 24, 1959, demonstrates placing a porous diaphragmcontaining a material which can be oxidized by metallic zinc between thepositive and negative electrodes to prevent the growth of the dendritesbeyond the separator. U.S. Pat. No. 3,226,260, issued to Drenglerdemonstrates a specially-designed separator which is permeable to gasand electrolyte. The oxygen evolved at the positive electrode permeatesthe separator and as the dendrites grow to where they touch theseparator, the zinc is oxidized and dissolved into the electrolyte.

These separators have not proved to be reliable. The porous nature ofthe separators is subject to fouling and contamination which severelyhampers the operation of the separators.

in place of using separators, it has been suggested to impart motion tothe zinc electrode. The turbulence of the electrolyte solution on theface of the zinc electrode retards columnar or dendritic growth;however, with a disc electrode, rotation of to l,400 revolutions perminute is required to retard the growth of dendrites and produce asomewhat uniform deposit. The rotation of the disc electrode can bereduced to l revolution per minute when the face of the zinc electrodeis wiped by a relatively soft, elastic material such as rubber (see forexample U.S. Pat. No. 3,440,098). The rubbing action bends the dendriteformations over and prevents them from shorting out the cell; however, avery rough and uneven zinc deposit is obtained. This uneven, roughsurface hampers the efficiency of the cell during discharge and furtherincreases dendrite growth during subsequent recharging. As a result, thezinc cell deteriorates with each charge and discharge cycle (similarlyto the shape change phenomena mentioned above) becoming useless after arelatively few cycles.

Attempts have also beenmade to minimize shape change including theaddition of binders such as Teflon and a reduction in the amount ofmercury present in the electrode (used to prevent self-discharge of thezinc electrode and evolution of hydrogen during periods of nonuse).

The efforts represented by the prior art suggestions for reducingdendrite growth from the zinc electrode have concentrated solely onpreventing the ultimate growth of a dendrite from the zinc electrode tothe other electrode with subsequent shorting out of the cell. In each ofthe prior art methods mentioned above, columnar growth is retarded orprevented from extending past a barrier situated between the electrodesso that zinc electrodeposition 'can take place without shorting out ofthe cells; but the prevention of dendritic growth does not in itselfimprove the rate of zinc electrodeposition, it only allows theelectroplating to proceed without shorting of the cells. It is,therefore, an object of this invention to provide a method whereby theformation of dendritic growth during recharging of an electrochemicalcell is prevented and in addition the rate of electrodeposition ismarkedly increased resulting in a cell having the capacity to berecharged very quickly.

Another object of this invention is to produce a thick (greater than lmil and even up to 20 or 30 mil) deposit on the zinc electrode of azinc-air or other zinc-electrode cell during the recharging of the cell.

A further object of the present invention is to produce a zinc electrodecell which can be discharged and recharged over a great many cycleswithout affecting the efficiency of the cell.

SUMMARY The present invention is directed towards an improvement inzinc-air cells and other rechargeable electrochemical cells whereby theformation of dendrites on the zinc or similar metal electrode during therecharging cycle is prevented, shape change of the zinc electrode iseliminated and the rate of zinc or similar metal electrodeposit ismarkedly increased. These improvements are obtained by continuously andrepetitively contacting the surface of the zinc electrode by what istermed herein as dynamically hard particles. By the term dynamicallyhard is meant the combination of the hardness of the particle, thecontact pressure of the particle on the surface of the surface of theelectrode and the speed at which the particle is moving relative to themetal layer surface is such as to produce an abrasive action on suchsurface sufficient to remove any electrodeposit forming a high spot onthe metal layer surface and to mechanically activate the surface.Activating" the surface of the electrode surface within the meaning ofthe present invention means so treating the surface to improve theutilization of current to deposit metal in sound adherent form ratherthan as powder and dendrites and appears to involve the removal of anypolarization layer and reaction product layer from the electrode surfaceand the disarrangement of the atoms in the electrode surface layer to adegree sufficient to cause increased activity on the electrode surface.The activation of the electrode surface results in a markable increasein the rate of electrodeposition of zinc or similar metal on theelectrode and prevents the development of dendritic formations on thesurface of the electrode.

The process of activation of the surface of an electrode is fullydescribed in my copending application, Ser. No. 34,500, filed May 4,1970, entitled Electrodeposition, now US Pat. No. 3,619,384 which inturn is a continuation-in-part of application Ser. No. 718,468, filedApr. 3, 1968 now abandoned. The entire contents of the copendingapplication are incorporated herein by reference.

The activation of the surface of the zinc electrode results in anincreased rate of electrodeposition which is reflected in the ability ofthe zinc-air cell to be recharged rapidly during the recharging cycle.The abrasive action prevents the formation of dendrites on the surfaceof the zinc electrode, and the zinc is electrodeposited in the form of auniform smooth, dense coating on the electrode thereby eliminating theshape change" problem associated with prior art cells. In addition, thepresent invention provides a method for obtaining a greater thickness ofzinc during recharging than obtainable with prior art cells. Thus, therecharged cell of the present invention has an improved larger capacityor greater ability to store electrical energy than the prior art zinccells.

DRAWINGS The present invention is'explained hereinafter in greaterdetail by reference to the accompanying drawings which show thepreferred embodiments of this invention. It should be understood,however, that the drawings and examples are given for purposes ofillustration only and that the invention in its broader aspects is notto be limited thereto.

IN THE DRAWINGS FIG. 1 is a cross-sectional view of a zinc-air cellwhich incorporates one preferred embodiment of the abrasive-activatingmeans of this invention.

FIG. 2 is a cross-sectional view of the zinc-air cell of FIG. 1 takenalong line 22 of FIG. 1.

FIG. 3 is a perspective view partly in section of theabrasive-activating disc of FIG. 1.

FIG. 4 is a cross-sectional top view of a set of electrodes from azinc-air cell showing a second preferred embodiment of the abrasivemeans of the present invention.

FIG. 5 is a cross-sectional view of a zinc-air cell according to theinvention wherein the air anode and the abrasive-activating means havebeen combined into one unit.

FIG. 6 is a perspective view of an electrode couple from a zinc-air cellaccording to this invention wherein the zinc anode is made of aplurality of sections of rectangular cross section which can be rotatedin such a manner that the face of the anode can be revolved so that theanode faces the abrasive-activating means during the recharge of thebattery.

FIG. 7 is a vertical section of another set of zinc-air cell electrodesincorporating the present invention wherein the zinc anode and theabrasive-activating means both take the form of a plurality ofcylinders.

FIG. 8 is a horizontal section taken along line 8-8 of FIG. 7.

FIG. 9 is a cross-sectional view of one embodiment of theporous-activating means of this invention.

FIG. 10 is a cross-sectional view of a composite battery composed ofthree separate cells incorporating the present invention.

FIG. 11 is a schematic diagram showing cells similar to those of FIG. 1arranged in series.

In its broadest embodiment, this invention provides for the improvementin rechargeable electrochemical cells wherein the cell electrode, whichfunctions as an anode during discharge and a cathode during recharge, iscontacted by an abrasive-activating means during the recharge cycle. Theabrasive-like action activates the surface of the electrode resulting inthe ability of the cell to be rapidly recharged. In addition, theabrasiveactivating means further prevents the formation of dendriticgrowth on the electrode during recharging. Thus, there is achieved acell which has the. capability of being rapidly recharged and whichinherently prevents the formation of dendrites on the face of theelectrode during recharging of the cell. I

In a preferred embodiment, the invention provides for a zinc-air cellhaving the capacity to be recharged rapidly without the formation ofdendritic deposits on the zinc electrode during recharge. While theabovementioned drawings and the following description are directed to azinc-air cell, it will be understood that other electrochemical activematerials can be employed. For example, tin, nickel, lead, silver andother metals may be substituted for the zinc, and the alkaline systemsdescribed for the deposition of zinc may be replaced by acid systemswhen other active materials are to be deposited.

Referring now to FIGS. 1 and 2, there is shown an electrochemical systememploying the present invention. The system comprises a cell 10containing a suitable electrolyte, and a set of zinc electrodes 11suspended from bus bars 12. Gas-depolarized electrodes 13 are suspendedfrom bus bar 14. Abrasive-activating pads are attached to a back-upplate 16 which in turn is attached to shaft 17.

Shaft 17 can be rotated as well as moved longitudinally along its axis.During normal discharge of the battery, the shaft 17 is moved to such aposition that the pads 15 and back-up plate 16 are spaced equidistantbetween anodes 11 and gas electrodes 13 and out of contact with eitheranodes 11 and gas electrodes 13. Bus bar 14 consists of a hollow tube ofconducting material which supplies air to the gasdepolarized electrodes13 through the hollow suspend ing bars 18. The electrochemical reactionstaking place on the zinc anode and air-depolarized electrode in thepotassium hydroxide zincate electrolyte are well described in the art.Zinc is dissolved to give zinc ions from the anode 11 and hydroxyl ionsare formed at the air-depolarized electrode 13. When supplying air tothe electrode 13, the nitrogen and other inert gases contained in theair are chemically inactive and must be vented from the cell throughvent 19.

During the recharging cycle, the shaft 17 is moved longitudinally sothat the abrasive-activating pads 15 are brought into contact with theface of electrodes 11. The shaft 17 is then rotated about its axis byappropriate means not shown in FIG. 1. The shaft 17 imparts rotation tothe back-up plate 16 and abrasive-activating pads 15. A source of directcurrent is connected to bus bars 12 and 14 in such manner that the zincelectrode 11 is supplied with negative current and the gas electrode 13is supplied with positive current. Metallic zinc is plated from thezincate, potassium hydroxide electrolyte on the zinc electrode 11 andoxygen is liberated at the gas electrode 13.

As explained above, the activation of the surface of electrode 11 isbelieved produced by mechanically distorting the surface of the face ofthe electrode. The amount of surface distortion or mechanical work is afunction of the speed of the hard particles and the pressure exerted bythe hard particles against the face of the electrode 11. At higherspeeds less pressure is required and, likewise, at slower speeds morepressure is required. Sufficient speed and pressure requirements caneasily be determined for the particular electrodes being activated. Theprocess of activation is fully described in my copending applications,Ser. No. 34,500, now U.S. Pat. No. 3,619,384 and Ser. No. 863,499, nowU.S. Pat. No. 3,619,401 filed May 4, 1970 and Oct. 3, 1969, respectivelyand abandoned application Ser. No. 718,468, filed Apr. 3, l968, theentire contents of which are incorporated herein by reference.

The gas or air-electrodes 13 preferably contain catalytic substancessuch as silver, gold and platinum embedded in a porous structure such asdemonstrated in U.S. Pat. No. 3,462,307. Other catalyst-containingporous plate electrodes known in the art can be employed in the practiceof this invention.

Auxiliary electrodes serving for charging the electrochemicallyreversible zinc electrode 11 can be used instead of the gas-depolarizedelectrodes. For example, the back-up plate 16 could be made of nickel,stainless steel and iron. A source of positive electricity could besupplied to plates 16 through shaft 17.

The present invention may also make use of rechargeable orelectrochemically reversible cathodes (anodes during recharge step) ofthe type conventionally employed coupled with anodes (cathodes duringrecharge step) such as silver/silver-oxide, and oxidized nickelelectrodes.

The depolarizing gases used in conjunction with the porous gaselectrodes can include halogens such as chlorine and fluorine inaddition to oxygen and air.

The abrasive-activating medium comprises a porous supporting matrixcontaining a plurality of small, dynamically hard, relatively inflexibleparticles held in substantially fixed, spaced relationship to oneanother. The process requires relative motion during the recharge stepbetween the electrode which is being plated and the activating medium.In addition, sufficient pressure is applied to the activating medium ina direction normal to the surface of the electrode sufficient tomechanically activate the surface. This means the removal of anypolarization layer and reaction product layer from the electrode and thedisarrangement of the atoms in the electrode surface to a degreesufficient to cause increased activity. The surface distortion producedby the present process amounts to introducing activity-enhancing defectsin a substantially uniform manner in the electrode surface such as wholeor partial dislocations, vacancies, stacking faults, twins, latticedistortions and the like.

Fresh electrolyte is supplied to the electrode surface throughentrapment by the porous-activating medium. The porous medium sweepsfresh electrolyte over the surface of the electrode. This sweepingserves also to carry away depleted electrolyte or unwanted productsresulting from the electrochemical reaction.

By the term particle as is used herein, is meant not only completelyseparate and discrete three-dimensional bodies, but also larger bodieswith a plurality of points, tips, projections or the like thereon as forinstance a relatively hard resinous coating on a fiber wherein thecoating contains multiple irregular spaced projections and is generallyuneven in nature.

The matrix used to support the activating particles iselectrolyte-permeable and is also somewhat compressiwe and deformable sothat it can be conformed to irregular surfaced electrodes and associateddeposits where necessary. The matrix must have a plurality of liquidentrapping or sweeping members which define small compartments or poreswhich function much like a bucket conveyor in carrying small quantitiesof electrolyte over the activated electrode surfaces. Many variations ofporous supporting matrices can be used, e.g., open mesh screens withactivating particles adhered to the mesh, non-woven articles bothcompressed and uncompressed, open cell foam sheets with the activatingparticles incorporated in or on the foam cell walls. Examples ofproducts which can be used in the present invention as activating mediaare illustrated in U.S. Pat. No. Re 2 l ,852 to Anderson which shows anopen-mesh product having abrasive grains adhered thereto; in U.S. Pat.No. 3,020,139 to Camp et al. which illustrates non-woven webs having aplurality of hard particles adhered to and along the web fibers; in U.S.Pat. No. 3,256,075 to Kirk et al. which illustrates a sponge containinghard resin-impregnated sponge particles; and in U.S. Pat. No. 3,334,041to Dyer et al.

which illustrates a coated abrasive product having perforations throughwhich electrolyte can flow. In this latter instance, the product must bemodified for the present process by making it non-conducting, i.e., itessentially becomes a standard coated abrasive product withelectrolyte-passing holes therethrough.

Referring now to FIG. 3, there is illustrated the abrasive-activatingpad 15 and back-up plate 16 of FIG. 1. The pad 15 is adhered to back-upplate 16. Back-up plate 16 can be made of any material which is inert tothe caustic electrolyte such as nickel, iron or mild steel. The back-upplate 16 contains passageways 30 which are so shaped and positioned thatwhen the back-up plate 16 is rotated liquid electrolyte will be pumpedthrough the passageways 30 and into the pad 15. Also, the hub section ofback-up plate 16 has a plurality of narrow baffles 31 fixed to theback-up plate 16 and extending therefrom. The pad 15 is circular inshape and has a section cut from its center so that the pad fits snuglyover the baffles 31 and against the side of backup plate 16. The baffles31 are arranged such that when the back-up plate rotates they pumpelectrolyte solution into the pad 15. The distance which the baffles 31extend from back-up plate 16 is somewhat smaller than the width of thepad 15.

FIG. 9 shows a highly enlarged and idealized portion of one type ofactivating media which is, in addition to those mentioned above,suitable for use in the present invention and illustrates the hardparticle-connecting matrix relationship. Reference numeral 25 representsfibers of a non-woven web (non-conducting fibers such as poly[ethyleneterephthalate] or the like) which are anchored one to the other at theirpoints of intersection by an adhesive binder 26. A plurality of small,hard, discrete particles 27 are positioned on the fibers 25 and in thepresent illustration are held to such fibers by the adhesive 26. Atleast some of the fibers 25 extend relatively parallel to the zincelectrode 11 as shown at 28 to form the thin-walled cells or electrolytesweeping members referred to above. (For purposes of illustration, theactivating particles 27 are here shown at some distance from theelectrode 11 although in operation of the present process they would bein contact therewith.)

FIG. 4 demonstrates another embodiment of the present invention. Thereis shown a top, cross-sectional view of a cell having a plurality ofZinc electrodes 41 and gas-depolarized counterelectrodes 43. Theabrasive-activating means takes the form of continuous belts 44. Theactivating means is composed of the hard particle-containing matrix asin FIG. 9. The activating belts 44 are extended between rollers 42 whichalso drive the belt in a continuous circular motion. The belts 44 comeinto contact with the zinc electrodes 41 and during recharge of the cellperform the same function as the pads of FIG. 1. That is, theabrasive-activating belts 44 activate the surface of the zinc electrode41 thereby markedly increasing the rate ofzinc electrodepositionthereon, and in addition the belts 44 mechanically prevent dendritesfrom extending from the zinc electrode 41 to the counterelectrodes 43.The rollers 42 are shown driven by motor 45.

During discharge of the cell, the belts 44 can be held stationary orrotated at a very slow speed. If the belt 44 is sufficiently porous theslow movement during discharge of the battery is not needed; however, amore efficient distribution of fresh electrolyte on the face of zincelectrode 41 will be obtained by slowly moving the belt 44 duringdischarge of the battery.

During the charge cycle, the belts 44 are driven at a rate sufficient tomechanically activate the face of the electrode 41 and supply the freshelectrolyte needed to the face of electrode 41. Sufficient tension mustalso be applied to belts 44 to exert sufficient pressure upon the facesof electrodes 41. As discussed above, for the apparatus in FIG. 1, thespeed of belt 44 and the pressure exerted on the electrode surface 41are selected to produce sufficient mechanical work to result inactivation of the electrode 41.

FIG. 5 shows another embodiment of the present invention wherein theabrasive-activating means and the air electrode are incorporated into asingle element.

The cell comprises zinc electrodes 51 suspended from bus bar 52. Acombined activating means and air electrode 53 consists of a porousabrasive-activating layer 54 having a cross section similar to thatactivating medium shown in FIG. 9 bonded to a porous electricalconducting layer 55 which is in turn bonded so as to make electricalcontact with the back-up plate 56. The back-up plate is attached to arotatable and longitudinally movable shaft 57. A circular opening 58running axially along the center of shaft 57 supplies air through adistributing system 59 in back-up plate 56 t0 the porous electricalconducting layer 55. The electri-' cal conducting layer is made from thesame materials as the gas electrode 13 of FIG. 1.

The operation of the apparatus of FIG. 5 is similar to the operation ofthe apparatus of FIG. 1. During normal discharge of the battery, thecombination activating means and air-electrode 53 is positioned so thatthe activating layer 54 is not in contact with the zinc elec trode 51.Air is supplied to the electrical conducting layer 55 and theelectrochemical reactions discussed above for the apparatus of FIG. 1take place at the zinc and air electrodes.

During recharge of the cell 50, the shaft 57 is moved longitudinally sothat the activating layer 54 of the combined activating means and airelectrode 53 is brought into contact with the surface of zinc electrode5 1. The shaft 57 is also given a rotational movement. A source ofnegative current is supplied to the zinc electrodes 51 and positivecurrent to back-up plate 56 through shaft 57. The electrodeposit of zincis then accomplished in the same manner as discussed above for theapparatus of FIG. 1.

FIG. 6 shows still another embodiment of the present invention whereinthe zinc electrode is composed of a plurality of slats 60 which canrevolve about their individual axis. The slats 60 are suspended by rolls61 which in turn are suitably connected to shaft 62 in such a manner sothat as shaft 62 is rotated, the slats 60 revolve 180. On one side ofthe zinc electrode there is suspended an air electrode 63 which issimilar to the air electrode 13 of FIG. 1. On the other side of the zincelectrode there is situated a rotatable abrasive-activating means 64consisting of a back-up plate 65 and a hard particle containing porousmatrix layer 66. The abrasive-activating means is similar to theabrasive-activating means 15 and 16 of FIG. 1.

During normal discharge, the zinc surface of electrode slats 60 face theair electrode 63. During recharge the electrode slats 60 are revolved sothat the zinc surface faces the abrasive-activating means 64. Theabrasive-activating means is advanced toward the electrode slats 60until the porous matrix 66 makes contact with the zinc surface ofelectrode slats 60. The abrasive-activating means is rotated by suitablemeans not shown and a source of current is supplied to the electrodeslats 60 and back-up plate 65. Zinc is electrodeposited on the surfaceof slats 60 in a manner similar to that process described forelectroplating the zinc electrode of FIG. 1.

electrode cylinders 72 and displaced to one side of the line joining thecenter of cylinders 72 in such a manner that the porous matrix, hardparticle material 73 on the surface of the cylinder 72 comes intocontact with the surface of the two cylinders 71 of which it isequidistance from. The cylinders 72 can also be moved away fromcylinders 71 so that no contact is made between cylinders 71 and 72. Thegas-depolarized electrode 74 is positioned near the zinc electrode 71 onthe side opposite from which the cylinders 72 are positioned.

During normal discharge of the battery, the cylinders 72 are moved awayfrom and out of contact with cylinders 71. The zinc dissolves into thecaustic electrolyte and reacts with oxygen at the gas-depolarizedelectrode 74.

During recharge, the cylinders 72 are moved towards cylinders 71 so thatthe porous abrasive matrix 73 comes into contact with the surface ofcylinders 71. Provision is made to rotate both cylinders 71 and 72 insuch a manner that the speed of the surfaces of cylinders 71 and 72 varyat least 15% from each'other. It is preferred to have cylinders 72rotating in the opposite direction to that which cylinders 71 arerotating. A

source of direct current is connected to cylinders 71 and 72 and zinc iselectroplated on the surface ofcylinders 71.

It will be recognized that the electrodes shown diagrammatically in thecell of FIG. 1 are in so-called parallel" arrangement. Such a call asshown in FIG. 1 is restricted to voltage of the zinc-air or otherelectrode couple used. It is thus a low voltage but high amperage typecell.

In FIG. 10, a battery is shown diagrammatically whereby the electrodepairs constituting the battery are separated fromeach other bypartitions 101. The individual electrodes are connected in series by busbars 102. The posts of the battery 104 and 105 have a voltage potentialequal to three times the voltage of a single zinc-air or other electrodeair used. In this arrangement, air must be supplied to the airelectrodes by a non-conducting supply duct 103.

The cell of FIG. 1 can be arranged in series with other like cells asshown schematically in FIG. 11. The cells, like those of FIG. 1, denotedby the number 110, are connected together in series by suitableelectrically conducting means 1 13. In this arrangement, air must besupplied to the air electrodes through supply means 111 which isconnected to electrodes 14 by an electrically'non-conducting connector112.

EXAMPLE A mild steel disc one-fourth by 3 inches was electroplated onone face with zinc at a current density of 505 amps per square foot. Thedisc was immersed into an electrolyte similar to the electrolyte used incommercial zinc-air batteries, that is a solution containing 34.5 gramsper liter of ZnO and 291.8 grams per liter of KOI-I. The surface of thedisc which received the electroplate was brought into contact with adisc similar to the activating disc of FIG. 1. The activating disc wasrotated at 500 revolutions per minute. A pressure of 2-4 pounds persquare inch was used to press the activating disc against theelectroplated disc.

The porous matrix surface of the activating disc was made from anon-woven nylon material bonded together as shown in FIG. 9 with apolyurethane resin. The bonded matrix was impregnated with 600 mesh SiCparticles.

A source of direct current was connected to the mild steel disc and theback-up plate of the activating disc. After five minutes of plating azinc plate 5 mils thick was adhered firmly to the face of the steel discwhich had been in contact with the porous abrading disc. The zincdeposit was smooth, hard and evenly plated on the steel disc. The rateof zinc deposition during recharge of prior art zinc-air batteries isapproximately 0.05 to 0.1 mil per minute.

It can be seen that a zinc-air battery according to this invention canbe recharged at a rate 10 to 20 times faster than as reported possiblein prior art batteries. The zinc deposit obtained according to thisinvention does not exhibit dendrite formation on its surface, it issmooth, dense, hard and adheres firmly to the electrode to which it hasbeen plated. A thick zinc deposit can be obtained on the zinc electrodeduring recharge of the battery resulting in a battery having anincreased capacity with respect to the prior art batteries. The uniform,dense, smooth, hard surface formed on the zinc electrode during rechargeallows the cell to be recharged many times more than the prior artbattery without any decrease in efficiency.

The electrodes of the present invention are not subject to failure byshape change as are the electrodes of prior art cells, and therefore,there need be no reduction in the mercury content of the electrodes asdone in prior art cells to reduce the shape change" effect. The presentelectrodes can contain the normal amount of mercury necessary to preventself-discharge of the zinc cell.

What is claimed is:

1. A rechargeable electrochemical power supply comprising:

a. a body of aqueous electrolyte;

b. an electrically conductive cathode permeable to the passage of airand oxygen therethrough while being substantially impermeable to'thepassage of electrolyte, said cathode having a surface in electrolyticcontact with said electrolyte and a means for supplyingoxygen-containing gas to the interior of said cathode;

c. a conductive anode material in electrolytic contact with saidelectrolyte;

. a supporting medium having a plurality of spaced small hard particlesdisposed between said anode and said cathode;

e. means for establishing relative motion and contact between thesurface on said anode material and the particles supported by saidsupporting means whereby the said entire surface of said anode materialis continuously mechanically activated at least during any rechargecycle;

f. electrical terminal means respectively electronically connected tosaid anode material and said cathode for interconnection to anelectrical load; and

g. means for interconnecting a source of electrical power to saidterminals whereby the power supply is recharged for subsequent reuse.

2. A rechargeable power supply as in claim 1 wherein the conductiveanode material is zinc.

3. A rechargeable power supply as in claim 2 wherein the supportingmedium containing the spaced particles thereon makes contact with thezinc anode only during the recharge cycle and a means is supplied towithdraw said supporting medium from the surface of the zinc anodeduring the period in which power is supplied to an external load.

4. A rechargeable power supply as in claim 2 wherein said supportingmeans comprises an electrolyte-penneable matrix having said plurality ofsmall particles adhercd to in fixed spaced relationship one to theother.

5. A rechargeable power supply as in claim 4 wherein said matrixcomprises a porous non-woven web.

6. A rechargeable power supply as in claim 4 wherein said matrixcomprises an open-weave fabric.

7. A rechargeable power supply as in claim 6 wherein said particlescomprise abrasive grains.

8. In a rechargeable electrochemical current generator including areversible electrode and a counterelectrode, said reversible electrodehaving a working surface of active material confronting saidcounterelectrode, said active material being prone to develop growthformations extending toward said counterelectrode during recharge, thecombination therewith of a supporting medium having a plurality ofspaced small hard particles thereon interposed between said reversibleelectrode and said counterelectrode, means for imparting relative motionand contact between the working surface on said reversible electrode andthe particles on said supporting medium at least during recharge wherebysaid entire working surface is continuously and repetitivelymechanically activated and said supporting means continuously preventssaid growth formations from forming on said working surface.

9. A rechargeable generator as in claim 8 wherein the reversibleelectrode is zinc.-

10. A rechargeable generator as in claim 9 wherein the supporting mediumcontaining the spaced particles thereon makes contact with thereversible electrode only during the recharge cycle and a means issupplied to withdraw said supporting medium from the working surface ofthe reversible electrode during the period in which the generator produc es c urrsnt.

1. A rechargeable electrochemical power supply comprising: a. a body ofaqueous electrolyte; b. an electrically conductive cathode permeable tothe passage of air and oxygen therethrough while being substantiallyimpermeable to the passage of electrolyte, said cathode having a surfacein electrolytic contact with said electrolyte and a means for supplyingoxygen-containing gas to the interior of said cathode; c. a conductiveanode material in electrolytic contact with said electrolyte; d. asupporting medium having a plurality of spaced small hard particlesdisposed between said anode and said cathode; e. means for establishingrelative motion and contact between the surface on said anode materialand the particles supported by said supporting means whereby the saidentire surface of said anode material is continuously mechanicallyactivated at least during any recharge cycle; f. electrical terminalmeans respectively electronically connected to said anode material andsaid cathode for interconnection to an electrical load; and g. means forinterconnecting a source of electrical power to said terminals wherebythe power supply is recharged for subsequent reuse.
 2. A rechargeablepower supply as in claim 1 wherein the conductive anode material iszinc.
 3. A rechargeable power supply as in claim 2 wherein thesupporting medium containing the spaced particles thereon makes contactwith the zinc anode only during the recharge cycle and a means issupplied to withdraw said supporting medium from the surface of the zincanode during the period in which power is supplied to an external load.4. A rechargeable power supply as in claim 2 wherein said supportingmeans comprises an electrolyte-permeable matrix having said plurality ofsmall particles adhered to in fixed spaced relationship one to theother.
 5. A rechargeable power supply as in claim 4 wherein said matrixcomprises a porous non-wOven web.
 6. A rechargeable power supply as inclaim 4 wherein said matrix comprises an open-weave fabric.
 7. Arechargeable power supply as in claim 6 wherein said particles compriseabrasive grains.
 8. In a rechargeable electrochemical current generatorincluding a reversible electrode and a counterelectrode, said reversibleelectrode having a working surface of active material confronting saidcounterelectrode, said active material being prone to develop growthformations extending toward said counterelectrode during recharge, thecombination therewith of a supporting medium having a plurality ofspaced small hard particles thereon interposed between said reversibleelectrode and said counterelectrode, means for imparting relative motionand contact between the working surface on said reversible electrode andthe particles on said supporting medium at least during recharge wherebysaid entire working surface is continuously and repetitivelymechanically activated and said supporting means continuously preventssaid growth formations from forming on said working surface.
 9. Arechargeable generator as in claim 8 wherein the reversible electrode iszinc.